You can keep giving the electrolytes & vitamins (S-A-C) as long as you want, but most people stop after a couple of weeks. The ACV at a 1/4 teaspoon per quart will be fine, as long as your waterer is not metal. You don't want to use too much because the sour taste can make the chicks drink less water. As they get older, you can increase.
my chicks are 8-12 weeks old and I have two waterers one has sav-a-chick in it and the other is plain it's starting to get hot here it's been in the 90's all week and i think of it as chicken gatorade and it really helps my weak roo that got smacked around by my dog when he was free ranging due to being a mean roo
I used sav a chik for 2 days for the first three I got that were very stressed from shipping. I gave it to them for 3 days until they seemed pretty active. When the replacement chicks from that order came in a week later, they were much more lively than the first crew (they had a day less of transit time) and I didn't add any sav a chik. I would have used it if anybody seemed run-down, but didn't feel they needed it. They are all just under 6 weeks old now and are doing well.
I would go on your instincts and watch the chicks. I don't suspect there'd be any harm in spiking their water for longer, as long as you are mixing it weak anyway. But if it's half strength, they probably aren't getting much benefit from it. If you have a packet left, go ahead and use it up, but if everyone's perky, I wouldn't run out and buy more. Just my opinion.